Multi-level power amplifier

ABSTRACT

A high efficiency, multiple power output power amplifier uses a pair of amplifiers having similar characteristics coupled to a pair of radio frequency RF couplers. When both amplifiers are operating, the power output is the sum of the outputs of the two amplifiers. When lower power operation is desired, one of the amplifiers is turned off and a high impedance is presented to the isolated port of the output RF coupler, thereby ensuring that all of the power output of the remaining power amplifier is available for output.

CROSS REFERENCE TO RELATED APPLICATIONS

This application relates to and further describes other aspects of theembodiments disclosed in the following co-pending and commonly assignedU.S. Patent application, and is incorporated by reference in itsentirety.

U.S. patent application Ser. No. 09/686,440, “HIGH EFFICIENCY MULTIPLEPOWER LEVEL AMPLIFIER,” Attorney Reference Number: 99RSS386, filed onOct. 10, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to maximizing the efficiency of radiofrequency power amplification in a wireless communication devicetransmitter, and, more particularly, to a high efficiency multi-levelpower amplifier.

2. Related Art

With the increasing availability of efficient low cost electronicmodules, mobile communication systems arc becoming more widespread. Forexample, there are many variations of communication schemes in whichvarious frequencies, transmission schemes, modulation techniques andcommunication protocols are used to provide two-way voice and datacommunications in a hand-held telephone-like wireless communicationhandset. While the different modulation and transmission schemes eachhave advantages and disadvantages, one common factor is the need forhighly efficient power amplification. As these communication devicesbecome smaller, the benefits provided by highly efficient poweramplifiers becomes increasingly important. One significant concern whendeveloping these handheld communication devices is power consumption. Ahigh efficiency power system decreases the amount of power consumed,thereby maximizing the power source life of the device.

Another concern in these wireless devices is the size of the circuitry.In order to minimize the size of the wireless communication device, itis desirable to integrate as much functionality as possible into fewerand fewer circuit modules. This enables the wireless communicationdevice to be smaller. Integrating components also provides the benefitof less power consumption. Smaller wireless communication devices aremore desirable by consumers in the marketplace.

Most power amplifier systems employed in wireless communication devicesmust operate efficiently over a broad range of operating power levels.Efficient operation is inherently difficult to achieve without complexcircuitry and logic to control the power amplifier(s). Typically,additional circuitry residing on a control die must be used to controlthe power amplifier circuit. However, this additional circuitry requiresadditional space, thereby making the wireless communication devicelarger, and utilizes additional power. Also, this circuitry addsadditional cost to each unit.

One conventional manner to achieve high efficiency power amplificationover a broad range of power output levels uses radio frequency (RF)switches to select different power amplifiers based upon the requiredpower output demand. Each of the power amplifiers is optimized for highefficiency at different power levels. Unfortunately, this techniquerequires the use of an additional control die to house the RF switches.The control die results in an additional cost per unit, increases thesize of the wireless communication device, and also consumes additionalpower.

Another conventional manner to achieve high efficiency poweramplification over a broad range of power output levels involves twoseparate amplifiers, each amplifier having different characteristics andeach amplifier optimized for high efficiency at different power levels.In such an arrangement, the amplifiers could be activated separatelywith separate control dies to satisfy the required power levels. a hatis, only one of the two power amplifiers is on at any given time.Microwave couplers may be used to ensure the correct phase match betweenthe two amplifiers. Unfortunately, this technique still requires aseparate control die. Furthermore, the two different amplifiers musthave a matched phase supplied at their input, thereby requiring that themicrowave couplers be extremely stable.

Therefore, there is a continuing effort in the industry to develop awireless power amplification circuit that achieves highly efficientpower amplification over a broad range of output power levels and thatis economical to mass produce in high volume.

SUMMARY

The invention provides a high efficiency multi-level power amplifierthat maximizes power amplifier efficiency and minimizes the requiredcontrol circuitry. Thus, the invention increases the power efficiency ofa power amplifier circuit by integrating many of the power amplifiercomponents and control circuitry onto an integrated circuit (IC). Also,the integration of all components onto a single IC simultaneouslyminimizes the amount of control circuitry required to control theamplifier, thereby allowing a reduction in size of the wireless device.Furthermore, the single IC may reduce the manufacturing cost of eachwireless device.

The high efficiency multi-level power amplifier utilizes two amplifiershaving different amplification characteristics that are connected to twoRF couplers. The power amplifier circuit uses both amplifiers when powerdemand is high and uses the output of a single power amplifier whenpower demand is lower. By utilizing power amplifiers having differentamplification characteristics, the output of the wireless device whenoperating in the low power operating condition is set to an optimizedlevel for a low power operation mode and is set to another optimizedlevel for a high power operation mode.

In another embodiment, the multi-level power amplifier is configuredsuch that the isolated port of each RF coupler is connected to animpedance modification circuit. When using only one power amplifier, theimpedance modification circuit eliminates the impedance mismatch causedby the single power amplifier operation by using an externally biasedsemiconductor to present the proper impedance to the coupler connectedto the inactive power amplifier. In this manner, any impedance mismatchbetween the operative and inoperative power amplifiers is compensated,thus allowing the single operating power amplifier to achieve optimalperformance. Related practices of operation and computer readable mediaare also provided.

Other systems, methods, features and advantages of the invention will beor become apparent to one with skill in the art upon examination of thefollowing figures and detailed description. It is intended that all suchadditional systems, methods, features, and advantages be included withinthis description, be within the scope of the invention, and be protectedby the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The components in the figures are not necessarily to scale, emphasisinstead being placed upon clearly illustrating the principles of theinvention. In the figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 is a block diagram illustrating selected components of a portablecommunication device.

FIG. 2 is a block diagram illustrating an embodiment of the multi-levelpower amplifier of FIG. 1.

FIG. 3 is a block diagram illustrating an embodiment of the multi-levelpower amplifier.

FIG. 4 is a block diagram illustrating an embodiment of the multi-levelpower amplifier having an impedance modification circuit.

FIG. 5 is a schematic view illustrating, in further detail, anembodiment of the impedance modification circuit of FIG. 4.

DETAILED DESCRIPTION 1. Overview

Although described with particular reference to a transceiver employedin a wireless communication device, the high efficiency multi-levelpower amplifier can be implemented in any system where it is desirableto have both high and low power amplification, and where the low poweramplification operating point is not equal to 50% of the high levelpower amplification operating point. Furthermore, the high efficiencymulti-level power amplifier can be implemented in software, hardware, ora combination of software and hardware. In at least one embodiment,selected portions of the high efficiency multi-level power amplifier areimplemented in hardware and software. The hardware portion of theinvention can be implemented using specialized hardware logic. Thesoftware portion can be stored in a memory and be executed by a suitableinstruction execution system (microprocessor). The hardwareimplementation of the high efficiency multi-level power amplifier caninclude any or a combination of the following technologies, that are allwell known in the art: a discrete logic circuit(s) having logic gatesfor implementing logic functions upon data signals, an applicationspecific integrated circuit having appropriate logic gates, aprogrammable gate array(s) (PGA), a field programmable gate array(FPGA), etc.

Furthermore, the high efficiency multi-level power amplifier software,that comprises an ordered listing of executable instructions forimplementing logical functions, can be embodied in any computer-readablemedium for use by or in connection with an instruction execution system,apparatus, or device, such as a computer-based system,processor-containing system, or other system that can fetch theinstructions from the instruction execution system, apparatus, or deviceand execute the instructions. In the context of this document, a“computer-readable medium” can be any means that can contain, store,communicate, propagate, or transport the program for use by or inconnection with the instruction execution system, apparatus, or device.The computer readable medium can be, for example but not limited to, anelectronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system, apparatus, device, or propagation medium. Morespecific examples (a nonexhaustive list) of the computer-readable mediuminclude the following: an electrical connection (electronic) having oneor more wires, a portable computer diskette (magnetic), a random accessmemory (RAM), a read-only memory (ROM), an erasable programmableread-only memory (EPROM or Flash memory) (magnetic), an optical fiber(optical), and a portable compact disc read-only memory (CDROM)(optical). Note that the computer-readable medium could even be paper oranother suitable medium upon which the program is printed, as theprogram can be electronically captured, via for instance opticalscanning of the paper or other medium, then compiled, interpreted orotherwise processed in a suitable manner if necessary, and then storedin a computer memory.

2. Example Environment

FIG. 1 is a block diagram illustrating selected components of a portablecommunication device 100. Wireless communication device 100 includes aspeaker 102, an optional display 104, a keyboard 106, and a microphone108, ail connected to baseband subsystem 110. For convenience ofillustration, connections between components in the baseband subsystem110 and the speaker 102, display 104, keyboard 106 and microphone 108are not shown in detail. However, one skilled in the art will readilyunderstand the detailed connection requirements that connect the abovecomponents to the baseband subsystem 110. In a particular embodimentwireless communication device 100 can be, for example but not limitedto, a portable telecommunication handset such as a mobile cellular-typetelephone.

Speaker 102 and display 104 receive signals from baseband subsystem 110via connections 112 and 114, respectively. Similarly, keyboard 106 andmicrophone 108 supply signals to baseband subsystem 110 via connections116 and 118, respectively. Baseband subsystem 110 includes at least amicroprocessor (μP) 120, a memory 122, analog circuitry 124, and adigital signal processor (DSP) 126 in communication via bus 128. Bus128, although shown as a single bus, may be implemented using multiplebusses connected as necessary among the subsystems within basebandsubsystem 110.

Microprocessor 120 and memory 122 provide the signal timing, processingand storage functions for wireless communication device 100. Analogcircuitry 124 provides the analog processing functions for the signalswithin baseband subsystem 110. Baseband subsystem 110 provides controlsignals to radio frequency (RF) subsystem 130 via connection 132.Although shown as a single connection 132, the control signals mayoriginate from DSP 126 or from microprocessor 120, and are supplied to avariety of points within RF subsystem 130. It should be noted that, forsimplicity, only the basic components of wireless communication device100 are illustrated. Detailed operation of these individual componentsare not described in detail other than to the extent necessary tounderstand the operation and functioning of these components withrespect to the invention. One skilled in the art will realize that awireless communication device 100 or other system employing themulti-level power amplifier 200 (see also FIG. 2) may have the componentshown in FIG. 1 connected in a different order and manner than shown inFIG. 1, or may not include all of the components shown in FIG. 1, or mayinclude additional components connected in some alternative manner withthe component shown in FIG. 1. Any such variations of a wirelesscommunication device 100 or other system that utilizes the multi-levelpower amplifier 200 are intended to be within the scope of thisdisclosure and to be protected by the accompanying claims.

Baseband subsystem 110 also includes analog-to-digital converter (ADC)134 and digital-to-analog converters (DACs) 136 and 138. ADC 134, DAC136 and DAC 138 communicate with microprocessor 120, memory 122, analogcircuitry 124 and DSP 126 via bus 128. DAC 136 converts the digitalcommunication information within baseband subsystem 110 into an analogsignal for transmission to RF subsystem 130 via connection 140. DAC 138provides a reference voltage power level signal to the two amplifiers214 and 216 residing in the multi-level power amplifier 200, viaconnections 142 and 144, respectively. Connection 140, shown as twolines having directed arrows, includes the information that is to betransmitted by RF subsystem 130 after conversion from the digital domainto the analog domain.

RF subsystem 130 includes modulator 146. Modulator 146, after receivinga frequency reference signal, also called a local oscillator signal orLO from synthesizer 148, via connection 150, modulates the analoginformation connection 140 and provides a modulated signal viaconnection 152 to upconverter 154. Upconverter 154 also receives afrequency reference signal from synthesizer 148 via connection 156.Synthesizer 148 determines the appropriate frequency that upconverter154 upconverts the modulated signal on connection 152. The modulatedsignal on connection 152 may be any modulated signal, such as, but notlimited to, a phase modulated signal or an amplitude modulated signal.Furthermore, it is possible to supply a phase modulated signal toupconverter 154 and to introduce an amplitude modulated signal componentinto multi-level power amplifier 200 through the power amplifier'scontrol channel. Most modulation techniques benefit from the inventiondescribed below.

Upconverter 154 supplies the modulated signal via connection 158 tomulti-level power amplifier 200. Multi-level power amplifier 200amplifies the signal on connection 158 to different power levels whilemaintaining a high efficiency level. Multi-level power amplifier 200amplifies the modulated signal on connection 158 to the appropriatepower level for transmission via connection 160 to antenna 162.Illustratively, switch 164 controls whether the amplified signal onconnection 160 is transferred to antenna 162 or whether a receivedsignal from antenna 162 is supplied to filter 166. The operation ofswitch 164 is controlled by a control signal from baseband subsystem 110via connection 132.

A portion of the amplified transmit signal energy on connection 160 issupplied via connection 168 to power control element 170. Power controlelement 170 forms a closed power control feedback loop and, if desired,supplies an AM component of the transmit signal via control channelconnection 172 to multi-level power amplifier 200.

As described above, a signal received by antenna 162, at the appropriatetime determined by baseband system 110, is directed via switch 164 toreceive filter 166. Receive filter 166 filters the received signal andsupplies the filtered signal on connection 174 to low noise amplifier(LNA) 176. Receive filter 166 is a bandpass filter that passes allchannels of the particular cellular system that the wirelesscommunication device 100 is operating. As an example, in a 900 MHz GSMsystem, receive filter 166 passes all frequencies from 935.1 MHz to959.9 MHz, covering all 124 contiguous channels of 200 kHz each. Thepurpose of this filter is to reject all frequencies outside the desiredregion. LNA 176 amplifies the very weak signal on connection 174 to alevel that downconverter 178 translates to a baseband frequency.Alternatively, the functionality of LNA 151 and downconverter 178 cap beaccomplished using other elements, such as, but not limited to, a lownoise block (LNB) downconverter.

Downconverter 178 receives a frequency reference signal, also called alocal oscillator (LO) signal from synthesizer 148, via connection 180.This LO signal instructs the downconverter 178 as to the properfrequency to downconvert the signal received from LNA 176 via connection182. The downconverted frequency is called the intermediate frequency or“IF”. Downconverter 178 sends the downconverted signal via connection184 to channel filter 186, also called the “IF filter”. Channel filter186 filters the downconverted signal and supplies it via connection 188to amplifier 190. The channel filter 186 selects the one desired channeland rejects all others. Using the GSM system as an example, only one ofthe 124 contiguous channels is actually to be received. Alter allchannels are passed by receive filter 166 and downconverted in frequencyby downconverter 178, only the one desired channel appears precisely atthe center frequency of channel filter 186. The synthesizer 148, bycontrolling the local oscillator frequency supplied on connection 180 todownconverter 178, determines the selected channel. Amplifier 190amplifies the received signal and supplies the amplified signal viaconnection 192 to demodulator 194. Demodulator 194 recovers thetransmitted analog information and supplies a signal representing thisinformation via connection 196 to ADC 134. ADC 134 converts these analogsignals to a digital signal at baseband frequency and transfers it viabus 128 to DSP 126 for further processing.

3. Multi-Level Power Amplifier

FIG. 2 is a block diagram illustrating an embodiment of the multi-levelpower amplifier 200 of FIG. 1. RF source 202 (FIG. 2) is illustrated forconvenience as an oscillator that includes the functionality ofmodulator 146 and upconverter 154 (FIG. 1) RF source 202 supplies amodulated and upconverted signal via connection 158 to input coupler204. Input coupler 204 is a four-port asymmetric quadrature hybridcoupler. Alternatively, input coupler may be any other type of wellknown RF coupling device capable of splitting and combining RF energy.Input coupler 204 includes a through port 206 and a coupled port 208.The through port 206 imparts a 0° phase shift on the signal onconnection 210 and the coupled port 208 imparts a −90° phase shift onthe signal on connection 212. The through port 206 of input coupler 204connects to an input of first amplifier 214 via connection 210. Thecoupled port of input coupler 204 connects to an input of secondamplifier 216 via connection 212.

Amplifiers 214 and 216 each receive a direct current (DC) bias signalvia connections 142 and 144, respectively, from the DAC 138 (FIG. 1).The bias signal communicated over connections 142 and 144 may bedifferent signals, or may be the same signal, depending on theparticular application that the invention is employed. Amplifier 214 andamplifier 216 have different operating characteristics. The DC biassignals provided from DAC 138 over connections 142 and 144 control theoperation of amplifiers 214 and 216, respectively, and determineswhether the amplifiers 214 and 216 are on or off. First amplifier 214connects, via connection 218, to the through port 220 of output coupler222. Second amplifier 216 connects via connection 224 to the coupledport 226 of output coupler 222. Output coupler 222 is also a four-portasymmetric quadrature hybrid coupler, similar in function to inputcoupler 204. Output coupler 222 may be identical to the input coupler204, or may be configured differently depending upon the particularapplication that multi-level power amplifier 200 is employed.Alternatively, output coupler 222 may be any other type of well known RFcoupling device capable of splitting and combining RF energy.

The input to coupled port 208 is connected to a resistor 228 viaconnection 230. The output of coupled port 226 is connected to aresistor 232 via connection 234. Resistors 228 and 232 are connected toground. Resistors 228 and 232, known as terminating resistors, provide ahigh impedance Such that amplifier 214 operates more efficiently whenamplifier 216 is off. (The on and off functionality of amplifier 216 isdescribed below.)

As mentioned above, first amplifier 214 and second amplifier 216 havedifferent operating characteristics. When both amplifier 214 andamplifier 216 are operating, the output present at connection 160 issubstantially the sum of the outputs of each amplifier 214 and 216.However, there are conditions when it is desirable to provide less thanfull power output. For example, when conditions permit, it is desirableto have a lower power output to conserve power while still maintainingamplifier output efficiency. For example, wireless communication device100 (FIG. 1) may have been moved closer to the base station receiver(not shown). Since a lower strength communication signal would beadequate, output power could be reduced to conserve power. Secondamplifier 216 is turned off by changing the signal from the DC bias fromDAC 138 via connection 144. Thus, first amplifier 214 is operating aloneand providing the entire power amplification for the communicationsignal.

First amplifier 214 is selected based upon the particular operatingcharacteristics specified by the designer when the wirelesscommunication device 100 is to be operating in the low power operatingmode. Such operating characteristics may include quiescent currentspecifications and efficiency ratings at various power level outputs.

Second amplifier 216 is then determined based upon the desired operatingcharacteristics when the wireless communication device 100 is operatedin the high power mode. The sum of the characteristics of the firstamplifier 214 and the second amplifier 216 (such as quiescent currentand efficiency ratings) determine operating characteristics at the highpower operating mode. Second amplifier 216 then could be specified bythe designer.

One skilled in the art will realize that once specified, the firstamplifier 214 and the second amplifier 216 may be conveniently selectedfrom a plurality of standardized parts to economically facilitatemanufacturing and assembly. Alternatively, first amplifier 214 and/orsecond amplifier 216 may be specially fabricated amplifiers having theoperating characteristics specified by the designer.

Power efficiency and reduced size may be realized during the fabricationprocess by installing the input coupler 204, output coupler 222,resistors 228 and 232, first amplifier 214 and/or second amplifier 216onto a single printed circuit board (PCB), thereby creating a smallmodularized component that is easy to install into a wirelesscommunication device 100 (FIG. 1) or other similarly functioning device.Also, many of the elements above may be incorporated into a singleintegrated circuit (IC) chip, further facilitating a reduction in sizeof the wireless device 100.

4. Embodiment of a Multi-Level Power Amplifier

FIG. 3 is a block diagram illustrating an embodiment of the multi-levelpower amplifier 300. Multi-level power amplifier 300 is configuredsubstantially according to the multi-level power amplifier 200 of FIG.2. multi-level power amplifier 300 has a first amplifier 302, a secondamplifier 304, an input coupler 306, an output coupler 308, andtermination resistors 310 and 312.

An RF signal from RF source 202 is provided to input coupler 306 viaconnection 158. The amplified output RF signal is provided to the RFoutput via connection 160. The RF source signal is amplified in a highpower mode when first amplifier 302 and second amplifier 304 areamplifying the RF source signal, or in a low power mode when only firstamplifier 302 is amplifying the RF source signal. Amplifiers 302 and 304have an amplification factor of 1,000 (×1000). The amplification factorof amplifiers 302 and 304 approximately equates to a gain of 30 decibels(dB). Thus, an input signal to amplifier 302 and/or amplifier 304 isamplified by a factor of 1,000.

Asymmetric input coupler 306 has a coupling ratio of 81% between thethrough port 314 and the coupled port 316. Thus, 19% of the incidentpower passes through the through port 314 and 81% of the incident powerpasses through the coupled port 316. For example, if the RF sourcesignal delivered to input coupler 306 via connection 158 is equal to 1milliwatt (mW), the signal delivered to first amplifier 302 viaconnection 318 has an amplitude of 0.19 mW and the signal delivered tothe second amplifier 304 via connection 320 has an amplitude of 0.81 mW.Thus, the input coupler is a four-port asymmetric coupler thatasymmetrically divides the RF source signal into two asymmetric signalcomponents where the first signal component equals 19% of the RF sourcesignal and the second signal component equals 81% of the RF sourcesignal.

Output coupler 308 is configured substantially the same as the inputcoupler 306. However, output coupler 308 takes two signal components andcombines them into one signal, the amplified RF output signal providedto RF subsystem 130 (FIG. 1) via connection 160. Continuing with theexample above having the RF source signal equaled 1 mW, the 0.19 mWsignal on connection 318 is amplified by first amplifier 302 into a 190mW signal This 190 mW signal is delivered to through port 322 viaconnection 324. The 0.81 mW signal on connection 320 is amplified by thesecond amplifier 304 into an 810 mW signal. This 810 mW signal isdelivered to coupled port 326 via connection 328. Output coupler 308then combines the 190 mW signal and the 810 mW signal into a single RFoutput signal having an amplitude of 1,000 mW. This 1,000 mW RF outputsignal is transmitted on connection 160 out to the RF subsystem 130(FIG. 1). For convenience of explaining the functionality of themulti-level power amplifier 300 shown in FIG. 3, the above-describedsignal amplitudes are shown on FIG. 3. The operation of the multi-levelpower amplifier 300 as described above represents operation in the highpower mode.

When the multi-level power amplifier 300 is operating in the low powermode, the DC bias signal from DAC 138 (FIG. 1) provided over connection144 is modified such that the second amplifier 304 is turned “off.” Thatis, the 0.81 mW signal of the illustrated example above on connection320 is not amplified. Thus, the output of second amplifier 304 providedon connection 328 equals 0 mW. Because first amplifier 302 is operatingin the “on” condition, the output of first amplifier 302 on connection324 equals 190 mW (in the illustrative example where the RF sourcesignal amplitude equals 1 mW). Since there is no signal provided tocoupled port 326, the output of the output coupler 308 equals the signaldelivered to through port 322 only. Thus, the output of the multi-levelpower amplifier 300 is equal to 190 mW, and is delivered to RF subsystem130 (FIG. 1) via connection 160.

In the multi-level power amplifier 300 the input coupler 306 and theoutput coupler 308 are shown to be like coupler units. The even-modeimpedance (Zoe) equals 176.32 ohms. The odd-mode impedance (Zoo) equals14.18 ohms. As noted above, the coupled port imparts a 90 degree phaseshift, also known as electrical length (EL), to the RF signal. Otherembodiments may employ asymmetric couplers having different couplingratios, depending on the particular application requirements of thedevice that the invention is employed.

When the multi-level power amplifier 300 is operating in the low powermode (second amplifier 304 is off) terminating resistors 310 and 312 areused to limit current flowing through the coupled port 316 and thecoupled port 326. In the multi-level power amplifier 300 the terminatingresistors 310 and 312 are equal to 50 ohms. Terminating resistor 310 iscoupled to the input of coupled port 316 via connection 330 andterminating resistor 312 is coupled to the output of coupled port 326via connection 332.

5. Embodiment of a Multi-Level Power Amplifier Utilizing a MatchingImpedance

According to the multi-level power amplifier 300 shown in FIG. 3, whenthe second amplifier 304 is turned off, the apparent phase differencebetween the coupled ports 316 and 326, and the through ports 314 and322, along with the impedance mismatch caused by turning off secondamplifier 304, prevents the remaining operating amplifier 302 fromproviding its full power. For example, first amplifier 304 in such anarrangement might provide merely 20% to 25% of its possible poweroutput.

FIG. 4 is a block diagram illustrating an embodiment of the multi-levelpower amplifier 400 having an impedance modification circuit 418. Toimprove the operating efficiency of the multi-level power amplifier 300(FIG. 3), an impedance modification circuit 402 is connected to theisolated port of output coupler 404 via connection 406. The impedancemodification circuit 402 (to be described in further detail below withrespect to FIG. 4), operates in cooperation with amplifiers 408 and 410,so that when amplifier 410 is turned off, impedance modification circuit402 presents a very high impedance to the coupled port 412 of outputcoupler 404. The high impedance allows the remaining operating amplifier(amplifier 408) to efficiently provide its full output power viaconnection 160. In other words, by changing the impedance at the coupledport 412 of Output coupler 404, significantly more power generated byamplifier 408 is available at the through port 414 of output coupler 404via connection 160 than is possible without the impedance modificationcircuit 402. In this manner, the multi-level power amplifier 400operates efficiently at both high power output and low power output.

6. Impedance Modification Circuit

FIG. 5 is a schematic view illustrating, in further detail, anembodiment of the impedance modification circuit 402 (FIG. 4). Impedancemodification circuit 402 couples to the coupled port 402 of outputcoupler 404 (FIG. 4) via connection 406. A resistive element, such as aresistor 502 presents a load to the coupled port 412 (FIG. 4) of outputcoupler 404 at all times. In one embodiment, resistor 502 is equal to 50ohms. When both amplifiers 408 and 410 are operating, the diode 504 ofFIG. 5 is forward biased into a conductive state caused by a negativevoltage applied via connection 506 (from DAC 138 of FIG. 1).Alternatively, the diode 504 can be forward biased by the negativecomponents of the signal present on connection 406 if zero voltage isapplied via connection 506. Forward biasing the diode 504 connects theresistor 502 through bypass capacitor 508, that behaves as a shortcircuit for AC signals, and connection 510 directly to ground. In thismanner, an impedance resulting from resistor 502 is presented to thecoupled port 412 of output coupler 404 at connection 406. If a zero voltsignal is applied via connection 506, then any negative components ofthe signal present on connection 406 enables conduction through thediode 504.

When it is desirable to provide lower power from multi-level poweramplifier 400 (FIG. 4), amplifier 410 (FIG. 4) is switched off via acontrol signal from connection 144 (FIG. 4) and, simultaneouslytherewith, diode 504 is reverse biased by the application of a positivevoltage via connection 506 causing diode 504 to stop conducting. Whendiode 504 is reverse biased, an extremely high impedance is presented tothe coupled port 412 of output coupler 404 (FIG.43) via resistor 502 andinductor 512, that behaves as an open circuit to the AC signal onconnection 514. In this manner, all power generated by amplifier 408(FIG. 4) is available at the output port of output coupler 404 viaconnection 160 (FIG. 4).

Advantageously, the diode 504 and the amplifiers 408 and 410 (FIG. 4)can be implemented using the same manufacturing processing technology.For example, gallium arsenide (GaAs) heterojunction bipolar transistor(HBT) technology can be used to fabricate both the diode 504 and thepower amplifiers 408 and 410 on the same die or IC chip.

The input coupler 416 and the output coupler 404 (FIG. 4) form a“balanced amplifier” configuration. Under high power operation, bothamplifiers 408 and 410 operate together, yielding an output powerapproximately equal to the sum of their individual output powers. Underlow power operation, amplifier 408 and diode 504 are simultaneouslyswitched off so that a high impedance is presented to the coupled port412 of output coupler 404. This high impedance is fed back at thecorrect phase to the single remaining operating amplifier 408, thatallows the amplifier 408 to be presented with a matched 50 ohmenvironment. In this manner, the single remaining amplifier 408 operatesunder optimal load conditions and delivers a power level approximately3dB lower than that delivered when both amplifiers 408 and 410 areoperating. Although illustrated using a diode to modify the impedancepresented to the amplifier 408, other devices may be used to modify theimpedance. For example, it is feasible to use an RF switch, a fieldeffect transistor, or a bipolar device biased under different conditionsto modify the impedance.

7. Embodiment Employing Two Impedance Modification Circuits

Referring to FIG. 4, impedance modification circuit 418 is coupled tothe coupled port 420 of input coupler 416. The impedance modificationcircuit 418 connects via connection 422 to the coupled port 420 of inputcoupler 416. For convenience of illustration and to indicate thatimpedance modification circuit 418 is an optional element, impedancemodification circuit 418 is shown with dotted lines. In the absence ofimpedance modification circuit 418, a fixed resistance may be connectedto the coupled port 420 of input coupler 416. Impedance modificationcircuit 418 is similar in structure and operation to impedancemodification circuit 402 described above with respect to FIG. 4.

8. Other Embodiments

While various embodiments of the invention have been described, it willbe apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible that are within the scopeof this invention.

What is claimed is:
 1. A method for operating multiple output levelpower amplifiers, comprising the steps of: providing a communicationsignal to an input coupler residing in a multiple output level poweramplifier, the input coupler coupled to an input of a first poweramplifier having a first amplification, and the input coupler coupled toan input of a second power amplifier having a second amplification, tiefirst amplification being smaller than the second amplification,specifying an amplification mode for amplification of the communicationsignal; operating the multiple output level power amplifier in ahigh-power amplification mode such that the firs, power amplifier andthe second power amplifier are operating to amplify the communicationsignal in accordance with tile specified amplification mode; andoperating the multiple output level power amplifier in a low-poweramplification mode such that the first power amplifier is operating toamplify the communication signal in accordance with the specifiedamplification mode and the second power amplifier is off.
 2. The methodof claim 1, further comprising the step of providing a control signal tothe second power amplifier, the control signal having a first state suchthat the second power amplifier is off, and the control signal having asecond state such that the second power amplifier is on.
 3. The methodof claim 2, further comprising the steps of: modifying an outputimpedance to a first output impedance value of an output impedancemodification circuit when the second power amplifier is operating, theoutput impedance modification circuit coupled to an output of an outputcoupler coupled to an output of the first power amplifier and coupled toan output of the second power amplifier; and modifying the outputimpedance to a second output impedance value when the second poweramplifier is off.
 4. The method of claim 3, further comprising the stepof providing the control signal to the output impedance modificationcircuit, such that the output impedance is modified to the first outputimpedance value when the control signal is in the first state, and suchthat the output impedance is modified to the second output impedancevalue when the control signal is in the second state.
 5. The method ofclaim 4, further comprising the steps of: modifying an input impedanceto a first input impedance value of an input impedance modificationcircuit coupled to an input of the input coupler when the second poweramplifier is operating; and modifying the input impedance to a secondinput impedance value when the second power amplifier is off.
 6. Themethod of claim 5, further comprising the step of providing the controlsignal to the input impedance modification circuit, such that the inputimpedance is modified to the first input impedance value when thecontrol signal is in the first state, and such that the input impedanceis modified to the second input impedance value when the control signalis in the second state.
 7. A system for providing multiple outputamplification levels in multiple output level power amplifiers,comprising: an input coupler configured to receive a communicationsignal; an output coupler configured to provide an amplifiedcommunication signal to an antennae, a first amplifier having a firstamplification and coupled between the input coupler and the outputcoupler; a second amplifier having a second amplification and coupledbetween the input coupler and the output coupler, the secondamplification being greater than the first amplification; and acontroller providing a control signal to the second amplifier such thatwhen the control signal is in a first state the second amplifier isactivated so that a multiple output level power amplifier is operatingin a high-power amplification mode with the first amplifier and thesecond amplifier operating to amplify a communication signal, and suchthat when the control signal is in a second state the second amplifieris deactivated so that the multiple output level power amplifier isoperating in a low power amplification mode with the first amplifieroperating to amplify the communication signal and the second amplifieroff.
 8. The system of claim 7, wherein the input coupler is an inputasymmetric coupler configured to transmit a first signal portion of thecommunication signal to an input of the first amplifier and configuredto transmit a second signal portion of the communication signal to aninput of the second amplifier, and wherein the output coupler is anoutput asymmetric coupler that combines an amplified first signalportion from an output of the first amplifier with an amplified secondsignal portion from an output of the second amplifier when the multipleoutput level power amplifier is operating in a high power output mode.9. The system of claim 7, further comprising all output impedancemodification circuit coupled to the output coupler such that when thecontrol signal is in the first state the output impedance modificationcircuit has a first impedance value and such that when the controlsignal is in the second state the output impedance modification circuithas a second impedance value.
 10. The system of claim 9, furthercomprising a coupler coupled between the output impedance modificationcircuit and the controller so that the control signal specifies thefirst impedance value and the second impedance value.
 11. The system ofclaim 9, further comprising an input impedance modification circuitcoupled to the input coupler such that when the control signal is in thefirst state the input impedance modification circuit has a first inputimpedance value and such that when the control signal is in the secondstate the input impedance modification circuit has a second inputimpedance value.
 12. The system of claim 11, further comprising acoupler coupled between the input impedance modification circuit and thecontroller so that the control signal specifies the first inputimpedance value and the second input impedance value.
 13. A system forproviding multiple output amplification levels in multiple output levelpower amplifiers, comprising: means for providing a communication signalto an input coupler residing in a multiple output level power amplifier,the input coupler coupled to an input of a first power amplifier havinga first amplification, and the input coupler coupled to an input of asecond power amplifier having a second amplification, the firstamplification being smaller than the second amplification; means forspecifying an amplification mode for amplification of the communicationsignal; means for operating the multiple output level power amplifier ina high-power amplification mode such that the first power amplifier andthe second power amplifier are operating to amplify the communicationsignal in accordance with the specified amplification mode; and meansfor operating the multiple output level power amplifier in a low-poweramplification mode such that the first power amplifier is operating toamplify the communication signal in accordance with the specifiedamplification mode and the second power amplifier is off.
 14. The systemof claim 13, further comprising means for providing a control signal tothe second power amplifier, the control signal having a first state suchthat the second power amplifier is off, and the control signal having asecond state such that the second power amplifier is on.
 15. The systemof claim 14, further comprising: means for modifying an output impedanceto a first output impedance value of an output impedance modificationcircuit when the second power amplifier is operating, the outputimpedance modification circuit coupled to an output of an output couplercoupled to an output of the first power amplifier and coupled to anoutput of the second power amplifier; and means for modifying the outputimpedance to a second output impedance value when the second poweramplifier is off.
 16. The system of claim 15, further comprising meansfor providing the control signal to the output impedance modificationcircuit, such that the output impedance is modified to the first outputimpedance value when the control signal is in the first state, and suchthat the output impedance is modified to the second output impedancevalue when the control signal is in the second state.
 17. The system ofclaim 16, further comprising: means for modifying an input impedance toa first input impedance value of an input impedance modification circuitcoupled to an input of the input coupler when the second power amplifieris operating; and means for modifying the input impedance to a secondinput impedance value when the second power amplifier is off.
 18. Thesystem of claim 17, further comprising means for providing the controlsignal to the input impedance modification circuit, such that the inputimpedance is modified to the first input impedance value when thecontrol signal is in the first state and such that the input impedanceis modified to the second input impedance value when the control signalis in the second state.
 19. A system for controlling transmitter power,comprising: a wireless communication device having a multiple outputlevel power amplifier, the multiple output level power amplifier furthercomprising; an input coupler configured to receive a communicationsignal; an output coupler configured to provide an amplifiedcommunication signal to an antennae, a first amplifier having a firstamplification and coupled between the input coupler and the outputcoupler; a second amplifier having a second amplification and coupledbetween the input coupler and the output coupler, the secondamplification being greater than the first amplification; and acontroller providing a control signal to the second amplifier such thatwhen the control signal is in a first state the second amplifier isactivated so that the multiple output level power amplifier is operatingin a high-power amplification mode with the first amplifier and thesecond amplifier operating to amplify a communication signal, and suchthat when the control signal is in a second state the second amplifieris deactivated so that the multiple output level power amplifier isoperating in a low power amplification mode with the first amplifieroperating to amplify the communication signal and the second amplifieroff.
 20. The system for controlling transmitter power in a wirelesscommunication device of claim 19, wherein the wireless communicationdevice is a cellular telephone.